Almost conda pinning

[robertodr]

This does what #487 attempted to do. Since we can't seem to get the Conda+MKL example to work on Appveyor, we skip that one. Followed up in #497

• [x] Pin MKL to any of its 2018 versions (2019 seems to make trouble on CI)
• [x] Use Visual Studio 2017 image on Appveyor (rather than the "previous" version). Closes #475
• [x] Add stub of change log
• [x] Skip certain generators altogether for certain recipes. Mainly in the hope that it speeds up the Visual Studio lanes on Appveyor.
• [x] Document difference between skip_generators and failing_generators

Status

• [x] Ready to go

[robertodr]

Now running only tests in chapter 1 on appveyor, as agreed on chat. Follow-up PR will restore everything once the precious cache is safely in out hands.